Such boosters are well known in automobile technology and comprise in general an envelope having an axis of symmetry, separated sealingly by at least one movable partition structure into a front chamber connected permanently to a source of low pressure, and a rear chamber connected selectively to the front chamber or to a source of hitch pressure by a three way valve means actuated by a control rod capable of bearing, through the intermediary of the front face of a plunger, on the rear face of a thrust rod which is solid with a reaction disc, the plunger sliding in a bore in the movable partition, the three way valve comprising a valve member disposed in a tubular rear part of the movable partition and co-operating through an annular front face with a first valve seat formed on the plunger and with a second valve seat formed on the movable partition, a space being arranged between the two valve seats and communicating with at least one radial passage formed in the movable partition and opening out into the rear chamber, at least one axial passage being formed in the movable partition and openings into the front chamber, the annular front face of the valve member being urged in the direction of the valve seats by a valve spring.
Very many documents illustrate this type of booster, like for example the document GB-A-2 095 778. For a long time, improvements have been sought in the performance of the boosters, and in particular reductions in the response time, such as to obtain rapid actuation of the brakes when a force is applied to the brake pedal and also the return time, so that the driver will be able to control precisely the amount of his braking action.
Improvement of the response time is obtained by increasing the cross sectional area of the air flow passage at the level of the first annular valve seat, and generally by increasing its diameter. On the other hand, when a reduction in the return time of the booster is sought, this requires a reduction in the diameter of the second annular valve seat. These two improvements are thus in contradiction since in this design of booster, the valve seats are disposed substantially in the same plane and the diameter of the first valve seat is limited by, amongst other parameters, the diameter of the second valve seat.